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The interference pattern technique (IPT) consists of the coherent addition of the direct and reflected global navigation satellite systems (GNSS) signals in the receiving antenna. The detected power oscillates (fading), and the amplitude of these oscillations is very sensitive to the soil reflection coefficient at the specular reflection point. Therefore, variations of the reflection coefficient can be mapped, and thus dielectric constant variations, from which soil moisture can be retrieved. This work extends the use of the IPT technique from vertical polarization (V-Pol) to horizontal polarization (H-Pol). Moreover, the IPT equations are reformulated to facilitate the combination of dual-polarization retrievals. Simulations of the interference patterns at V- and H-Pol are presented for different soil moisture conditions. An upgrade of the SMIGOL GNSS-R instrument for dual-polarization observations is presented. This instrument was deployed in a flat, dry grassland in Yanco, Australia, in order to validate the proposed concepts. Finally, a comparison between the data retrieved from the SMIGOL instrument and the ground-truth soil moisture data is presented showing a good agreement between them and rainfall information.